Oxidation of metal–diethylenetriamine-pentaacetate (DTPA) – complexes during drinking water ozonation

This study investigates the oxidative transformation of diethylenetriaminepentaacetate (DTPA), a synthetic ligand, during drinking water ozonation. The rate coefficients for the reactions of CaDTPA3− and ZnDTPA3− with ozone were determined to be 6200 and 3500±150 M−1 s−1, respectively. The reactivity of Fe(III)DTPA2− towards ozone was found to be much lower (<10 M−1 s−1), but near neutral pH the reactivity of the Fe(III)-complexes is dominated by [Fe(III)(OH)]DTPA3−. For the reaction of Fe[(III)(OH)]DTPA3− with ozone a rate coefficient of 2.4±0.2×105 M−1 s−1 was measured. The rate coefficients of the reactions of the ZnDTPA-and Fe(III)DTPA with OH radicals have been determined by a competitive method as 2.4±0.4×109 and 1.5±0.1×109 M−1 s−1, respectively at pH=7. The degradation of low concentrations of DTPA complexes during ozonation was investigated in natural waters under drinking water relevant conditions. Based on our findings CaDTPA3− and ZnDTPA3− are judged as easily degradable. Fe(III)DTPA complexes showed a somewhat lower reactivity, but were still typically degraded by one order of magnitude at ozone dosages of 20 μM (1 mgL−1) in the three natural waters tested. Molecular ozone was found to be the major oxidant for the metal–DTPA complexes during ozonation.